There are a number of industrial processes and like applications that require precision control of a process fluid that is pumped, under pressure, through a process apparatus. By way of example only, the molding apparatus for molding of plastic articles may require a flow of high temperature process fluid, under pressure, to maintain the molding equipment at a precise desired temperature. In some applications, usually using water as the basic process fluid, the required temperature may be of the order of 200.degree. to 300.degree. F., depending upon the pressure employed and other process equipment requirements. In other instances, utilizing oil or other high temperature process fluids, the required temperature may be in a range of 400.degree. to 600.degree. F. The pressures employed may range up to 100 pounds per square inch or even higher. For effective quality control and high production rates, the process equipment may require that the fluid temperature be held to within a range of plus or minus 1.degree. F. or even less.
For the most part, controls used for these applications have concentrated on precise thermal measurements and have functioned in much the same manner as ordinary thermostatic controls, sometimes with safety controls based on maintenance of minimum pressure. Systems of this general kind are frequently inadequate in the precision of the temperature control that is exercised. In addition, they do not take sufficient account of other system changes, such as the interaction between pressure and temperature conditions, to provide for precise maintenance of the necessary overall process fluid operating conditions.
In most industrial applications utilizing high-temperature high-pressure process fluids, the pumping of the fluids is effected by pumps powered by three-phase electric motors. System operation is adversely affected if the pump operates in reverse from the design direction; moreover, the thermal controls ordinarily do not work properly under reverse pumping conditions. This difficulty can be overcome by directional starting controls for the three-phase motor, but controls of this kind are usually relatively expensive. Moreover, known systems have frequently been deficient with respect to assurance of minimum circulation conditions.